1. Field of the Invention
The present invention relates to a method of producing metal nanoparticles with a uniform size distribution and metal nanoparticles produced thereby, and particularly, to a method of producing metal nanoparticles continuously by using hot compressed alcohol or supercritical alcohol as a solvent.
2. Background of the Invention
Metal nanoparticles are expected to be applied to various industrial fields including electronic components/devices, catalysts, pigments, sensors, antimicrobial drugs, bactericides, condensers, paint, ink, magnetic tapes, electromagnetic wave absorbents, and medical devices, owing to their unique optical, magnetic, electrical, and chemical characteristics that are different from bulk metallic particles. Recently, the importance of these metal nanoparticles is being much emphasized.
Generally, methods for producing metal nanoparticles are categorized into a physical method and a chemical method.
The physical method includes a gas evaporation-condensation method of producing metal nanoparticles by first heating a metal under a vacuum or low pressure atmosphere thereby making the metal into gas, then by cooling and condensing the gas, an atomization method of producing metal nanoparticles by first melting a metal and then by dispersing the melted metal into gas using a spray, a mechanical grinding method using a ball milling, etc. Here, the gas evaporation-condensation method and the atomization method have an advantage to produce metal nanoparticles of a relatively uniform size and a high purity. However, the gas evaporation-condensation method and the atomize method are very expensive techniques and are not suitable for massive productions, due to complex device configurations, low yields and production rates, high energy consumption, and high production costs, etc. On the other hand, the mechanical grinding method is suitable for industrial massive productions. However, with the mechanical method, it is difficult to produce particles with high purity metal due to the impurity incorporation during the mechanical process, and it is difficult to produce uniform size metal nanoparticles due to the limitation of mechanical precision.
The chemical method includes a vapor phase reducing method of contacting vapor of a metal precursor, e.g., metal chloride with reducing agents such as hydrogen and carbon monoxide, etc., a liquid phase reducing method of producing particles by reducing a metal precursor using a reducing agent in organic solvents or aqueous solvents, an electrolysis method of producing metal nanoparticles by reducing metal ions in a solution with an alternating current or a direct current in the solution through a metal electrode, etc. Here, the vapor phase reducing method has a disadvantage that an expensive device such as a device using plasma or a chemical vapor evaporator is required. The liquid phase reducing method is a relatively simple process, but this method is not economical and often generate a large quantity of organic or aqueous wastes. In addition, the liquid phase reducing method relies on using a very toxic reducing agent and organic solvent such as hydrazine, formic acid, and boron compounds (NaBH4, LiBH4, KBH4). Accordingly, when applying produced metal nanoparticles to cosmetics, drugs, materials to be injected into a human's body, etc., the metal nanoparticles should be purified using a very time-consuming and high cost process. The liquid phase reducing method is not environmentally friendly since a large amount of liquid organic wastes are discarded. Furthermore, when a large volume batch reactor is used for massive productions, produced nanoparticles have very non-uniform size distributions due to non-uniformity of an inner temperature of the batch reactor or the metal precursor.
The electrolysis method has disadvantages that long production time is required, productivity is very low due to a low metal precursor concentration, and high costs are required in processing waste water since strong acids such as sulfuric acid are required to be used.
Accordingly, required is a method of producing metal nanoparticles continuously at a high rate and a high yield using a simple device without using an expensive or toxic reducing agent or strong acid.